When the system was set up, the first and last blocks were reserved, the remaining blocks were divided up into two groups A and B/C. Corporations like IBM got A blocks long ago. Now B and C blocks in the remaining upper half of the nnn.---.---.--- mask are are divided up among the rest of the world.

The first and last blocks of each subnet are also reserved, etc.

You can also find good summaries on most of the Network Interface card makers sites where you are instructed to on how to set up a network.

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